Process for producing ceramic substrates for microelectronic circuits

ABSTRACT

A ceramic substrate for densely integrated semiconductor arrays which is superior in a coefficient of thermal expansion, dielectric constant, strength of metallized bond, and mechanical strength, comprising a sintered body composed essentially of mullite crystals and a non-crystalline binder composed of SiO 2 , Al 2  O 3 , and MgO, is provided.

BACKGROUND OF THE INVENTION

The present invention relates to ceramic substrates for microelectroniccircuits and to a process for producing the substrates. Moreparticularly, the invention is directed to a ceramic substrate having aspecially low dielectric constant, low coefficient of thermal expansionand high mechanical strength and permitting wiring thereon with ahigh-melting metallic conductor, and to a process for producing suchceramic substrates.

In recent years, with the increasing integration degree of semiconductordevices, there are growing needs for circuit substrates supporting suchdevices to accept higher-density wiring and to have higher performancecharacteristics, higher reliability, and so forth. In particular,subjects important to circuit substrates for use in electronic computersand the like are high-speed signal propagation and high reliability. Forthese substrates, there are used, in practice, ceramics composed mainlyof alumina (Al₂ O₃).

Desired characteristics of the ceramic to be used for such circuitsubstrates are generally as follows:

(1) The ceramic insulator is dense and has a hermetic nature. Thismatter relates to the overall reliability of the circuit substrate.

(2) The coefficient of thermal expansion of the ceramic is as close aspossible to that of silicon chips. This is for the purpose of minimizingstrain which will develop at the junction between the ceramic substrateand the silicon chip to prolong the joint life and enhance thereliability.

(3) The dielectric constant of the ceramic is minimized. This is forspeed-up of the signal propagation.

(4) Junction of conductor metals to the ceramic substrate is strong,that is, the metallized bond strength is high. This relates to the bondstrength between the circuit substrate and the output or input terminal.

(5) The ceramic has a high mechanical strength. This is necessary forhandling in the process for fabricating the substrate and for mountingonto a sealing means and a cooling means to the substrate.

Thus the material to be used for the circuit substrates should satisfythe above requirements simultaneously. In particular, circuit substrateseach loaded with several tens densely integrated semiconductorcomponents for use in electronic computers will be inapplicablepractically if any one of the above items is not satisfied.

Conventionally Al₂ O₃ is used for substrates of this type. Although itis satisfactory in hermetic nature, metallized bond strength andmechanical strength, it has a higher coefficient of thermal expansion of8×10⁻⁶ /°C. than that of silicon chips (3×10⁻⁶ /°C.) and also has a highdielectric constant of about 10. Accordingly, Al₂ O₃ is not suitable forcircuit substrates.

Known ceramic insulators having a lower coefficient of thermal expansionand dielectric constant than that of Al₂ O₃ include silica (SiO₂, ε=ca.4), cordierite crystal (5SiO₂.2Al₂ O₃.2MgO, ε=ca. 5.0), cordierite glass(ε=6.3), steatite (MgO.SiO₂, ε=6.3), forsterite (2MgO.SiO₂, ε=6.5), andmullite (3Al₂ O₃.2SiO₂, ε=7).

However, the coefficient of thermal expansion of SiO₂ and cordieritecrystal are very low, i.e., as low as 5×10⁻⁷ /°C. and 1.5×10⁻⁶ /°C.,respectively, and those of steatite and forsterite are 7.2 and 9.8 (roomtemperature - 400° C.), respectively, which are nearly equal and higherthan that of Al₂ O₃. The coefficient of thermal expansion of cordieriteglass is about 3.7×10⁻⁶ /°C., which is close to that of silicon chips,but the mechanical strength of cordierite glass is as low as 100 MPa, sothat the cordierite glass is impractical for circuit substrate purposes.

Mullite is somewhat unsatisfactory in dielectric constant andcoefficient of thermal expansion, but it has a high mechanical strengthof 350 MPa, which is thus most promising among the conventionalceramics.

However, mullite has the following inherent problems (1) and (2):

(1) Bond strength between mullite and a usual conductor metal ismarkedly low. This is because no chemical reaction occurs betweenmullite and either tungsten (W) or molybdenum (Mo), which is usedcommonly as a conductor metal on alumina substrates and the like, evenat elevated temperatures. This property is inherent in mullite.

(2) Highly strengthening of the above-mentioned bond requires a specialpowder of mullite and a special sintering method which are impracticalas well as expensive. That is, K. S. Mazdiyasni and L. M. Brown["Synthesis and Mechanical Properties of Stoichiometric AluminumSilicate (Mullite)", J. Am. Ceramic Soc., 55[11], 548-555 (1972)]obtained a sintered mullite body capable of forming a high strength bycompacting a fine powder of mullite and sintering the compacted body ata temperature as high as 1800° C.

It is very difficult, however, to form such a powder in green sheets(before sintering), which are preforms of circuit substrates. Moreover,the sintering temperature of 1800° C. is much higher than those used forusual substrates, e.g., 1500° to 1650° C. This is a significantbottleneck in practicing this method in view of also the heatingelements and heat insulator of the furnace.

While mullite is inherently hard to sinter, as described above, therehas long been used a method referred to as "liquid phase sintering"which has been reduced into practice for producing sintered hard alloys.

The typical sintered hard alloy is composed of tungsten carbide (WC) andcobalt (Co). Although WC is difficult to sinter in single form, it canbe made into a high-density sintered body when burned jointly withseveral percentages of cobalt. This is because cobalt is melted in thesintering step and the melted cobalt draws WC in the solid phase theretoby the surface tension thereof.

This liquid phase sintering method is also applied to the sintering ofAl₂ O₃ for producing circuit substrates therefrom. That is, usual Al₂ O₃particles of several μm in size are hard to sinter, but they can bedensely sintered according to the liquid phase sintering mechanism byaddition of a material (an eutectic composition of three or fourcomponents such as SiO₂, Al₂ O₃, MgO, and CaO) fusible at a far lowertemperature than is Al₂ O₃.

In the above two examples, both cobalt and the three- or four-componenteutectic composition, which generate a liquid phase, play the role ofpromoting the sintering of a hardly sinterable substance. Nevertheless,the former is called a binder and the latter a sintering aid, ingeneral.

The reason for the above is as follows: In the case of the WC-Cosintered hard alloy, WC crystal grains are strongly bonded togetherthrough metallic cobalt, and the high hardness and high toughness ofthis alloy can be altered optionally with the combination of hard andbrittle WC and tough cobalt. Thus the binder function of cobalt is veryeffective.

In the case of the Al₂ O₃ circuit substrate, the sintering of Al₂ O₃ canbe greatly promoted by addition of the three- or four-component eutecticcomposition, but the original properties of Al₂ O₃ are scarcely variedby this addition. Therefore, the three- or four-component eutecticcomposition is generally called a sintering aid.

From the above described point of view, studies of sintering aids formullite have been made for the purpose of solving difficulties insintering mullite ceramics. Of course, these studies are all intended tomake denser the texture of mullite according to the liquid phasesintering mechanism by using cordierite as another sintering aid.

For instance, in Japanese Patent Laid-Open No. 139709/80 and in"Preparation and Properties of Mullite-Cordierite Composites" [B. H.Mussler and M. W. Shafer, Am. Ceram. Soc. Bull., 63, 705 (1984)],discussion is given on the use of mullite as a matrix and cordierite asa sintering aid.

From the equilibrium diagram of the SiO₂ -Al₂ O₃ -MgO system, it can beseen that the melting point of 5SiO₂.2Al₂ O₃.2MgO is 1490° C., which isfar lower than the melting point (1830° C.) of mullite. Thus the mullitetexture has been made denser by the liquid phase sintering action,yielding a sintered body of zero % water absorption.

While the sintering aid used in Japanese Patent Laid-Open No. 139709/80and the B. H. Mussler et al article are equally referred to ascordierite, it is not clear from the former whether the cordierite iscrystalline or amorphous, and B. H. Mussler et al use crystallinecordierite.

Cordierite either in a crystalline or amorphous form has a lowercoefficient of thermal expansion and a lower dielectric constant thanthose of mullite as stated above. Accordingly, it is expected that theaddition of cordierite to mullite will lower the coefficient of thermalexpansion and dielectric constant of mullite as well as produce thesintering promoting effect

In the Laid-Open No. 139709/80, a sintered body having a coefficient ofthermal expansion ranging from 4.2×10⁻⁶ to 3.8×10⁻⁶ /°C. and dielectricconstant ranging from 6.7 to 6.5 is obtained when the proportion ofcordierite to mullite is altered from 3.63 to 36.2% by weight.

In the Laid-Open No. 139709/80, while cordierite is incorporated into amullite crystal matrix, the composition range within which theabove-mentioned characteristics are obtained is expressed in terms ofMgO, Al₂ O₃ +SiO₂, and the weight ratio of Al₂ O₃ /SiO₂. Such expressionof composition is obviously inappropriate for sintered bodies madedenser by the liquid phase sintering mechanism and for sintered bodiesall the characteristics of which are dependent on Al₂ O₃ crystal matrix.It is reasonable to express the compositions of sinteredmullite-cordierite bodies in terms of the proportion of cordierite tomullite.

According to the article of B. H. Mussler et al., sintered bodies havinga coefficient of thermal expansion ranging from 4.5×10⁻⁶ to 3.2×10⁻⁶/°C. and dielectric constant ranging from 5.7 to 4.8 are obtained whenthe proportion of crystalline cordierite to mullite is altered from 17.1to 76.8% by weight.

In the two prior art examples described above, the obtained sinteredbodies, when used for circuit substrates, are nearly satisfactory in airtightness, coefficient of thermal expansion and dielectric constant.

The mechanical strength of ceramics, that is, one of the characteristicsrequired for circuit substrates is not described in the two prior artexamples. Hence, it is doubtful whether these prior art ceramics aresatisfactory in strength when used as circuit substrates.

Moreover, no result of investigation on metallized bond strength isdescribed in the prior art examples. Simultaneous aggregative sinteringof a conductor metal with an insulator ceramics is indispensableparticularly for fabricating multilayer circuits comprising a number ofsubstrates. Nevertheless, no description is given on the metallized bondstrength in the prior art examples. It is a fatal matter in using theseceramics for circuit substrates if the metallized bonds thereof areweak.

The reason for giving no result about the metallized bond strength inthe prior art examples may be that the sintering aids used in theexamples have fundamental defects which affect the metallizing ofmullite substrates.

Since mullite does not react chemically with any of such high-meltingmetals as W and Mo, the liquid phase penetration method that is appliedto Al₂ O₃ substrates and the like is indispensable in order to joinfirmly such metals with mullite.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a ceramic circuit substratewhich can overcome the above noted drawbacks of the prior art, has adense texture, coefficient of thermal expansion closest to that ofsilicon, sufficiently lower dielectric constant than that of Al₂ O₃, andhigh mechanical strength and can be joined firmly to such high-meltingmetals as W and Mo.

It is another object of the invention to provide a process for producingsuch ceramic substrates.

The present invention is based on the finding of a novel binder forsintering mullite which is satisfactory in any of the compacting actionbased on the liquid phase sintering mechanism, improvements of ceramicsin properties, and strong joining of ceramics to conductor metals by theliquid phase penetrating action.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a) and (b) illustrates the ability of a liquid to wet a solid;

FIG. 2 shows the relation of a SiO₂ content in binder to a melting pointof binder;

FIG. 3 shows the relation of a SiO₂ content in binder to a bindercontact angle on mullite;

FIG. 4 shows the relation of a SiO₂ content in binder to a bindercontact angle on each of W and Mo;

FIG. 5 shows the relation of a binder-to-mullite proportion to acoefficient of thermal expansion of mullite-binder sintered composition;

FIG. 6 shows the relation of a binder-to-mullite proportion to adielectric constant of mullite-binder sintered composition;

FIG. 7 shows the relation of a SiO₂ content in binder to a dielectricconstant of mullite-binder sintered composition;

FIG. 8 shows the relation of a binder-to-mullite proportion to aflexural strength of mullite-binder sintered composition;

FIG. 9 shows the relation of a SiO₂ content in binder to a flexuralstrength of mullite-binder sintered composition;

FIG. 10 shows the relation of a SiO₂ content in binder to a porosity ofmullite-binder sintered composition;

FIG. 11 shows the relation of a sintering temperature to a flexuralstrength for a mullite-binder composition;

FIG. 12 is a microscopic photograph showing the fine structure of aceramic substrate according to the present invention;

FIG. 13 is a microscopic photograph showing the fine structure of aceramic substrate-W conductor junction according to the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The basic concept of the present invention will be described below. Thebinder to be used in the present invention needs to meet the followingrequirements:

(1) In order to sinter mullite densely by the liquid phase sinteringmechanism, the melting point of the binder should be lower than that ofmullite, chemical reaction should occur slightly between mullite and thebinder at suitable sintering temperatures, and mullite crystal grainsshould be wetted sufficiently by the molten binder.

(2) In order to join a conductor metal to mullite by the liquid phasepenetrating action, the binder in a molten form mentioned in (1) aboveshould wet sufficiently the conductor metal.

(3) Similarly to the cobalt in the WC-Co alloy, the binder should haveeffects of improving mullite in properties such as a coefficient ofthermal expansion, dielectric constant, and mechanical strength.

To summarize the above requirements, the binder to be used inmullite-based circuit substrates needs to have a sufficiently lowermelting point than does mullite, the binder in a molten form should wetsufficiently the conductor metal and mullite crystal grains, and thebinder should contain much SiO₂, in other words, the matrix of thebinder should be SiO₂ which, among heat-resistant inorganic simplematerials, has the lowest coefficient of thermal expansion anddielectric constant.

Now description is given on the wettability, which has an importantconnection with the present invention. FIG. 1 illustrates shapes ofmolten droplets 2 placed separately on solid plates 1. In the figure, θis called contact angle. At θ>90° (FIG. 1(a)), it is said that the plateis not wetted, and at θ<90° (FIG. 1(b)), it is said that the plate iswetted. For the present invention, the conductor metals and mullitecrystal grains correspond to the solid plates, and the bindercorresponds to the molten droplets, in FIG. 1.

Generally, in order to densify a solid by the liquid phase sintering,the condition of θ≃90° is insufficient and the condition of θ≦50° isusually required. In consequence, the binder to be used for mullitecircuit substrates is desired to exhibit θ<50°.

The present invention is illustrated in more detail with reference tothe following examples.

EXAMPLE 1

FIG. 2 shows the relation of melting point to SiO₂ content (wt %)measured on the binder compositions according to the present inventionfor use in mullite circuit substrates. For the purpose of lowering thecoefficient of thermal expansion and dielectric constant of mullite byaddition of a binder, the SiO₂ content in the binder must be at least50% by weight, but with a 100% SiO₂ content a melting point is 1740° C.,which is too high to sinter materials for circuit substrates. Hence thebinder of 100% SiO₂ is impractical.

The principle of melting point depression by addition of second andthird elements to SiO₂ is utilized for the purpose of maintaining atleast 50% SiO₂ content in a binder and putting the melting point of thebinder within the usual sintering temperature range of 1550° to 1770° C.for circuit substrates metallizable with tungsten or molybdenum.

In view of the above, binders were prepared by altering the SiO₂ contentfrom 50 to 90 wt. %, as shown in FIG. 2, the Al₂ O₃ content from 35 to 4wt. %, and the MgO content from 15 to 1 wt. %.

Binders of these compositions were heated to 1650° C., droplets of theresulting molten binders were placed on plates of mullite sintered in asingle form, and the contact angles were measured. Results thereof areshown in FIG. 3. The contact angles are up to 50° so far as the SiO₂content in each binder lies in the range of 50 to 90 wt. %.

Further, droplets of the molten binders were placed on molybdenum andtungsten metal plates and the contact angles were measured. Resultsthereof are shown in FIG. 4, wherein curve 1 is on molybdenum and curve2 on tungsten.

Comparing the results shown in FIGS. 2, 3, and 4 with the foregoingrequirements for the binder to be used in mullite circuit substrates, itcan be seen that among the binder compositions shown in FIG. 2, thosecontaining 50 to 90 wt. % of SiO₂ exhibit contact angles of up to 50° onmullite and on molybdenum and tungsten metals at sintering temperaturesof 1550° to 1660° C. The contact angle of a binder is desired to be assmall as possible when the binder is used for sintering mullite,tungsten, or molybdenum. Therefore, it is concluded from FIGS. 3 and 4that the preferred range of SiO₂ contents in the binder is from 60 to 80wt. %.

In the next place, FIG. 5 shows coefficients of thermal expansion (roomtemperature - 500° C.) of mullite-binder sintered compositions, thecoefficients having a great influence on the junction between theresulting circuit substrates and silicon chips. The coefficients ofthermal expansion shown in FIG. 5 were of sintered bodies prepared bymixing mullite with various proportions of a binder having a definitecomposition (SiO₂ 90 wt. %, Al₂ O₃ 7.0 wt. %, MgO 3.0 wt. %) andsintering the mixtures at 1620° C. for 1 hour. From FIG. 5, desirableproportions of the binder to mullite are found to be from 10 to 30 wt.%.

As is clear from the above, the coefficient of thermal expansionincreases with the proportion of the binder to mullite. The coefficientof the sintered body containing no binder was measured by K. S.Mazdiyasni and L. M. Brown. Such a change in coefficient of thermalexpansion as shown in FIG. 5 is due to the larger coefficient of thebinder than that of mullite.

It is known that the coefficient of thermal expansion of a substancegenerally depends on the composition and crystal structure of thesubstance and the coefficient of thermal expansion of a substance in anoncrystalline or amorphous form is larger than that of the substance ina crystalline form. Examination revealed that the binder used in theabove-mentioned measurement of a coefficient of thermal expansion wasnoncrystalline and microscopically in a glass state. From these facts itmay be said that binders used in the present invention arenoncrystalline.

Then, coefficients of thermal expansion were measured similarly but byaltering the binder composition in the range shown in FIG. 2 whileconstantly maintaining the mullite-to-binder ratio within a range of75:25. The results indicated that the coefficient of thermal expansiondecreased from 5.7×10⁻⁶ /°C. to 4.8×10⁻⁶ /°C. as the SiO₂ content wasincreased.

According to the prior art example, i.e., Japanese Patent Laid-Open No.139709/80, the coefficient of thermal expansion of mullite is decreasedgreatly by adding cordierite as a sintering aid. This is considered toresult from the extremely smaller coefficient of thermal expansion ofcrystalline cordierite, used as the sintering aid, than that of mullite.

It can been seen from the foregoing that the coefficient of thermalexpansion of the ceramics according to the present invention is muchsmaller than that of the prior art Al₂ O₃ substrate and hence veryeffective in enhancing the reliability of junction between the circuitsubstrate and the Si chip.

FIG. 6 shows the measurements of dielectric constants (1 MHz) for thesame mullite-binder sintered compositions as used in FIG. 5. FIG. 7shows results of mullite-binder sintered compositions prepared byaltering the binder composition as shown in FIG. 2 while constantlymaintaining the mullite-to-binder ratio within a range of 75/25. FromFIG. 6 it seems that desirable binder-to-mullite proportions minimizethe dielectric constant; however, suitable values of said proportionsare from 10 to 30 wt. % in consideration of the balance between thedielectric constant and other properties of the sintered product. Whiledesirable binder compositions selected from FIG. 7 also seems to lowerthe dielectric constant, suitable SiO₂ contents for binders are from 60to 90 wt. %. In this composition range, the dielectric constant isstable without notable variation.

As shown in FIGS. 6 and 7, the dielectric constant decreases greatlyfrom the original value of mullite as the binder proportion and the SiOcontent in binders are increased. Of the dielectric constant values inFIG. 6, that of the composition containing no binder is measured by K.S. Mazdiyasni and L. M. Brown.

The effect of binders in ceramic substrates of the present invention,outlined above, is slightly inferior on the coefficient of thermalexpansion and dielectric constant of ceramics to the effect of sinteringaid used in the two prior art examples mentioned before. This is becausethe present invention employs noncrystalline binders while the prior artexamples employ crystalline sintering aids.

However, these slightly reduced coefficient of thermal expansion anddielectric constant do not matter in overall consideration ofcharacteristics required for circuit substrates. That is, the mostimportant subjects for putting a ceramic circuit substrate intopractical use are the mechanical strength of the ceramic substrate andthe bond strength between the ceramic substrate and the conductor metal,which are described below.

FIG. 8 shows the results of three-point bending tests for flexuralstrength, which is one of the characteristics required for circuitsubstrates, on the same samples as in FIG. 5. FIG. 9 shows the resultsof the above-mentioned bending tests on the same samples as in FIG. 7.Ceramics having flexural strengths of at least 15 kg/mm², which isnecessary for circuit substrates, are found from FIG. 8 to havebinder-to-mullite proportions of 10 to 35 wt. %, preferably 15 to 30 wt.%, and are bound from FIG. 9 to have SiO₂ contents of 50 to 95 wt. %,preferably 65 to 90 wt. %, in each binder.

Decrease, as shown in FIG. 8, in flexural strength when the binderproportion exceeds 30 wt. % seems to be caused by the lower strength ofthe binder itself than that of mullite. Also decrease, as shown in FIG.9, in flexural strength when the SiO₂ content in each binder exceeds 90wt. %, seems to be caused by the inhibition of mullite compaction by theincreased amounts of SiO₂.

One of the factors controlling the flexural strength of ceramics isinsufficient compaction in sintering ceramics which leaves pores insintered bodies. It is known that the strength of sintered bodiesincreases with decrease in the porosity thereof FIG. 10 shows theporosities of the same samples as in FIG. 9. As is seen from FIG. 10,the porosity is up to 5% when the SiO₂ content in each binder rangesfrom 60 to 90 wt. %; these results are well consistent with the flexuralstrengths shown in FIG. 9. Thus, the desirable range of SiO₂ contents isfrom 65 to 90 wt. %. Of course, the ceramics according to the presentinvention are hermetic in the range of binder compositions shown in FIG.10.

It is also known that the strength of composite materials like theceramics according to the present invention is generally much dependenton the difference in a coefficient of thermal expansion between thematrix and the binder, besides on the above-mentioned pores remaining inthe composite materials. When the coefficient of thermal expansion ofthe matrix is much larger than that of the binder, high tension isexerted on the binder in the cooling stage after sintering. Thisinternal stress causes a marked decrease in the strength of the entirecomposite material.

Considering the strength of the ceramics according to the presentinvention in the light of the above mentioned mechanism of decreasingthe strength, the coefficient of thermal expansion of the present binderis believed to be considerably close to that of the matrix mullite. Thisis due to the noncrystalline structure of the binder used in the presentinvention and is a natural consequence in consideration also of theforegoing example of cordierite, which indicates that the coefficient ofthermal expansion of a noncrystalline substance is considerably largerthan that of the crystalline substance.

Finally, explanation is made on the results of tests for theceramic-conductor metal junction, i.e., the metallized bond strength,which is an essential requirement for circuit substrates.

Since no chemical reaction occurs between ceramic mullite and eithertungsten or molybdenum even at such a high temperature as 1650° C. (in areducing atmosphere) as stated before, some amount of a liquid phase isnecessary, in other words, the liquid phase penetration mechanism mustbe utilized, in order to join these materials firmly.

Usually, multilayer circuit substrates have a structure in which ceramicinsulative layers and conductor metal layers are superposed alternatelyone upon another. For substrates of such a structure, it is ideal thatthe composition of the liquid phase for joining the ceramic to theconductor metal is identical with the composition of the binder forsintering the ceramic densely. The foregoing results shown in FIGS. 3and 4 reveal that the binder according to the present invention has asufficient wettability for both mullite and tungsten or molybdenum.Green sheets of ceramics were prepared from mixtures of mullite and 28wt. %, based on the mullite, each of binders having the samecompositions as shown in FIG. 4. Marks of 2 mm square were printed onthese green sheets with each of tungsten and molybdenum conductorpastes. The resulting sheets were sintered at 1630° C. for 2 hours toprepare specimens. These specimens were measured for the strength ofmetallized bonds. The bond strengths were 1.5 to 5 kgs for tungsten and1.0 to 4.0 kgs for molybdenum.

The metallized bond strength of circuit substrates is generally desiredto at least 1 kg. Hence, the ceramics according to the present inventionare found to be sufficient for practical use. The above-mentioned highmetallized bond strength is caused by nothing but the molten binder,according to the present invention, which has a small contact angle onmullite as well as on tungsten and molybdenum metals, thus fullyexhibiting the liquid phase penetrating effect.

The used paste is composed of a high-melting metal such as tungsten ormolybdenum, solvent, and organic vehicle. The mixing proportions ofthese three components vary somewhat depending upon the desiredconductivity. Generally the proportions are 70 to 85 wt. % of thehigh-melting metal, 10 to 29 wt. % of a solvent, and 1 to 5 wt. % of anorganic vehicle. Desirably the high-melting metal has an averageparticle size of 0.5 to 2.0 μm and a purity of at least 99.9%.

The capability of sintering simultaneously the ceramic insulator and theconductor is of great advantage in controlling a process for fabricationof electronic computer circuit substrates to be used for multilayerwiring, and in reducing production costs.

EXAMPLE 2

FIG. 11 shows the relation between flexural strength and sinteringtemperature examined on a mullite-based ceramic. Test specimens wereprepared from a mixture of 80 wt. % of mullite and 20 wt. % of a bindercomposed of 90 wt. % of SiO₂, 7 wt. % of Al₂ O₃, and 3.0 wt. % of MgO.The specimens were sintered at different temperatures for 60 minutes ina reducing atmosphere.

As is seen from FIG. 11, ceramics according to the present inventionsintered at a temperature of 1550° to 1700° C. have a flexural strengthof at least 15 kg/mm² which is required by circuit substrates. Preferredsintering temperatures is from 1600° to 1700° C. These results are alsoconnected to the binder-wettability of mullite and the like. It is amatter of course that the above suitable range of sintering temperaturesis restricted by the melting temperature of a binder having thecompositions as shown in FIG. 2 and by the contact angle of the binderon mullite at various temperatures.

EXAMPLE 3

Explanation is made below on the circuit substrates of the presentinvention, fabricated by the green-sheet lamination method.

A commercial mullite powder having an average particle size of 2 μm anda binder powder (having a particle size of 1-3 μm) composed of 60 wt. %of SiO₂, 30 wt. % of Al₂ O₃, and 10 wt. % of MgO are thoroughly mixed inrespective proportions of 70 wt. % and 30 wt. % by means of a wet typeball mill. An organic binder, plasticizer and dispersion medium areadded as compacting aids. The organic binder is polyvinyl butyral,acrylic ester or the like; the plasticizer is phthalic ester or thelike; the dispersion medium is alcohol, trichloroethylene or the like.

The slurry obtained by addition of these compacting aids is compactedby, for example, the doctor blade method. This method is carried out bycoating a slurry to a uniform thickness on a base film (carrier tape),drying the coat to solid, and separating the solid coat from the basefilm to yield a raw material sheet for ceramics, usually called a greensheet.

The thickness of the mullite-based sheets prepared in the above methodis from 0.15 to 0.25 mm so as to meet the dielectric constants requiredfor circuit substrates. Then holes are formed by punching or othersuitable ways through the green sheets for the purpose of later wiringthrough the multilayer substrates. These through holes are filled with atungsten paste prepared by adding a resin and a solvent to a tungstenmetal powder having an average particle size of 1 μm. The tungsten pasteused herein is composed of, for example, 77.5 wt. % of a tungsten powder(a purity of at least 99.9% and average particle size of 1±0.5 μm), 20wt. % of diethylene glycol mono-n-butyl ether acetate, 2.0 wt. % ofethyl cellulose, and 0.5 wt. % of polyvinyl butyral.

Then, intended patterns are printed with a similar tungsten paste on thegreen sheets for wiring of outermost circuit layers and inner circuitlayers. Molybdenum also may be used in place of tungsten for the paste.

20 ceramic green sheets thus wired are superposed one upon another, andhot-pressed under a pressure of about 50 kg/cm² at 110° C. to form alaminate of ceramic green sheets. This laminated body is sintered at1580° C. for 5 hours in a humidified atmosphere of hydrogen to yieldmullite-based multilayer circuit substrates wired with tungstenconductor.

The obtained circuit substrates exhibited a water absorption of zero %,coefficient of thermal expansion of 5.4×10⁻⁶ /°C., dielectric constantof 6.1, and flexural strength of 20 kg/mm². Cross sections of thesubstrates were observed with a microscope to examine the finestructure. As shown in FIG. 12, these substrates were found to have astructure in which mullite particles are surrounded with the binder. Thecharacteristic and effect of the present invention were confirmedreadily from FIG. 12.

EXAMPLE 4

Multilayer circuit substrates were prepared in the same manner as inExample 3 by forming green sheets from a mixture of 80 wt. % of mullite(the same powder as used in Example 3) and 20 wt. % of a binder (SiO₂ 90wt. %, Al₂ O₃ 7.0 wt. %, MgO 3.0 wt. %), forming through holes andcircuit patterns, laminating 25 resulting sheets, and sintering thelaminate at 1620° C. for 2 hours in a humidified atmosphere of hydrogen.

The obtained substrates exhibited a water absorption of zero %,coefficient of thermal expansion of 5.2×10⁻⁶ /°C., dielectric constantof 5.9, and flexural strength of 25 kg/mm². Cross sections of thesesubstrates were observed with a microscope to examine internal wiringbetween the substrate layers. It was confirmed therefrom that theceramic layers and conductor metal layers were united completely asshown in FIG. 13. This completely united state results from thedensification of mullite being performed in the sintering step by thebinder added and from the tungsten layers being penetrated sufficientlywith the binder.

EXAMPLE 5

Multilayer circuit substrates were prepared in the same manner as inExample 3 by forming green sheets from a mixture of 85 wt. % of mullite(the same powder as used in Example 3) and 15 wt. % of a binder (SiO₂ 95wt. %, Al₂ O₃ 4 wt. %, MgO 1 wt. %), forming through holes and circuitpatterns, laminating 18 resulting sheets, and sintering the laminate at1660° C. for 1 hour in a humidified atmosphere of hydrogen.

The obtained substrates exhibited a water absorption of zero %,coefficient of thermal expansion of 4.6×10⁻⁶ /°C., dielectric constantof 5.7, and flexural strength of 21 kg/mm². Input and output terminalsof Kovar metal were fixed with a brazing material on the upper surfaceof the substrates and a tensile test was conducted so as to apply stressbetween the substrate and each terminal. The result showed that allbreaks occurred within the ceramic substrate and the mode of breaks wassimilar to that of usual alumina substrates. This indicates that thesubstrates of this example are sufficient for practical use.

According to the present invention, it is possible to produce circuitsubstrates which are dense and superior in thermal and electricalproperties to conventional alumina substrates, that is, have very lowcoefficients of thermal expansion of 4.5 to 5.5×10⁻⁶ /°C. and very lowdielectric constants of 5.5 to 6.2, additionally have sufficientflexural strengths of 15 to 25 kg/mm², and can be sinteredsimultaneously with a conductor metal such as W and Mo for wiring to beunited therewith into a single body. Therefore, the present substrateshave such distinct effects that the signal propagation speed can beincreased by at least 25% as compared with conventional aluminasustrates, reliability also is hightened at the junctions between thesubstrate and the Si chip and between the substrate and either of inputand output terminals, the economy is made better, and the process isstabilized.

What is claimed is:
 1. A process for producing ceramic substrates whichcomprises the steps of;forming green sheets from a compositionconsisting of 70 to 85% by weight of a mullite ceramic and 30 to 15% byweight of a binder composed of 60 to 95% by weight of SiO₂, 4 to 30% byweight of Al₂ O₃, and 1 to 10% by weight of MgO; printing circuitpatterns with a conductor paste comprising a high-melting metal on thegreen sheets; and sintering the resulting sheets at a temperature of1550° to 1700° C. in a reducing atmosphere.
 2. The process for producingceramic substrates according to claim 1, wherein said high-melting metalis tungsten or molybdenum.
 3. A process for producing ceramic substrateswhich comprises the steps of;forming green sheets from a compositionconsisting of 70 to 85% by weight of a mullite ceramic and 30 to 15% byweight of a binder composed of 60 to 95% by weight of SiO₂, 4 to 30% byweight of Al₂ O₃, and 1 to 10% by weight of MgO; forming holes throughthe green sheets; printing circuit patterns with a conductor pastecomprising a high-melting metal on the green sheets; filling the throughholes with said conductor paste; laminating a plurality of the resultinggreen sheets to form a laminate; and sintering the laminate at atemperature of 1550° to 1770° C. in a reducing atmosphere.
 4. Theprocess for producing ceramic substrates according to claim 3, whereinsaid high-melting metal is tungsten or molybdenum.